This invention relates to a process for the production of ethyl acetate.
Etyl acetate is a relatively expensive bulk chemical which is conventionally produced by esterification of acetic acid with ethanol according to equation (1):
CH3.CO.OH+CH3CH2OHxe2x95x90CH3.CO.O.CH2.CH3+H2Oxe2x80x83xe2x80x83(1).
Because this reaction does not tend to lead to formation of by-products which have boiling points close to that of ethyl acetate, recovery of substantially pure ethyl acetate from the esterification product mixture is usually not complicated by the presence of by-products of the esterification reaction.
Some other methods which have been proposed for the conversion of ethanol to ethyl acetate, however, tend to lead to formation of by-products, notably n-butyraldehyde and butan-2-one, which have boiling points close to that of ethyl acetate and hence make the recovery of substantially pure ethyl acetate from the resulting reaction product mixtures more difficult than from esterification reaction mixtures. These methods include dehydrogenation of ethanol, oxidation of ethanol, reaction of ethanol with acetaldehyde, and oxidation of ethanol to acetaldehyde followed by the Tischenko reaction.
Ethyl acetate can be produced from acetaldehyde according to the Tischenko reaction given in equation (2):
2CH3.CHOxe2x95x90CH3.CO.O.CH2.CH3xe2x80x83xe2x80x83(2).
It is also possible to produce ethyl acetate from ethanol by dehydrogenation according to equation (3):
2CH3.CH2.OHxe2x95x90CH3.CO.O.CH2.CH3+2H2xe2x80x83xe2x80x83(3).
According to China Chemical Reporter, 26 Mar. 1996, a plant with a capacity of 5000 tonnes per annum for production of ethyl acetate by dehydrogenation of ethanol has been constructed at Linshu Chemical Fertilizer Plant of Shandong using a catalyst developed by Qinghua University.
Ethanol is produced in large quantity by a variety of processes, including hydration of ethylene, the Fischer Tropsch process, or as a fermentation product. The purity of the ethanol often depends upon the method used for its production. For example, although hydration of ethylene yields a substantially pure ethanol product, the Fischer Tropsch process yields also a number of by-products which are troublesome to remove from the ethanol product. In the case of fermentation, the ethanol product is obtained as an aqueous solution which may also contain by-products whose removal from the ethanol is difficult.
In certain circumstances ethanol may be available in excess capacity, whilst acetic acid is not readily available in the necessary quantity. Accordingly, there are many reasons why, particularly in countries having a relative abundance of ethanol with respect to acetic acid, it is commercially interesting to produce ethyl acetate from ethanol, acetaldehyde or a mixture thereof.
Catalytic dehydrogenation of alcohols with reduced copper under ultra violet light was described by S. Nakamura et al, in Bulletin of the Chemical Society of Japan (1971), Vol. 44, pages 1072 to 1078.
K. Takeshita et al described reduced copper catalysed conversion of primary alcohols into esters and ketones in Bulletin of the Chemical Society of Japan, (1978) Vol. 51(9), pages 2622 to 2627. These authors mention that the mechanism for ester formation has been described in the literature as the Tischenko reaction. That is to say that dehydrogenation of ethanol yields acetaldehyde as an intermediate which combines according to the Tischenko reaction to produce ethyl acetate. Alternatively, or as well, 1 mole of ethanol may combine with 1 mole of acetaldehyde to yield 1 mole of ethyl acetate and 1 mole of hydrogen according to equation (4)
CH3CH2OH+CH3.CHOxe2x95x90CH3.CO.O.C2.CH3+H2xe2x80x83xe2x80x83(4).
U.S. Pat. No. 4,996,007 teaches a process for the oxidation of primary alcohols to aldehydes, acids and esters, particularly to aldehydes. In this process a primary alcohol is contacted, together with molecular oxygen, with a catalyst selected from ruthenium, rhodium, platinum, palladium, rhenium and mixtures thereof, optionally a quaternary C1 to C20 alkyl ammonium cocatalyst, and as oxygen activator dihydrodihydroxynaphthalene, dihydrodihydroxyanthracene or a mixture thereof. The product aldehydes, acids and esters are then separated from the reaction mixture.
In U.S. Pat. No. 4,220,803 catalytic dehydrogenation of ethanol for the production of acetaldehyde and acetic acid using a supported copper oxide essentially free of barium is proposed.
A silver-cadmium alloy catalyst has been suggested for use in production of alkyl alkanoate esters, by contacting a primary alkanol in the vapour phase with the catalyst at a temperature of between about 250xc2x0 C. and 600xc2x0 C., in U.S. Pat. No. 4,054,2424.
In U.S. Pat. No. 4,440,946 there is described a process for producing a carboxylate ester which comprises contacting a mixture of alcohol and aldehyde in the vapour phase with a coprecipitate composition comprising silver-cadmium-zinc-zirconium which is substantially in the free metal form.
Use of the Tischenko reaction for the production of mixed esters from aldehydes is described in U.S. Pat. No. 3,714,236.
U.S. Pat. No. 5,334,751 teaches production of ethyl acetate by reaction of ethanol and oxygen in the presence of a solid catalyst that contains crystalline TiP2O7 and has the formula PdaMbTiPcO7, where M is Cd, Au, Zn, Tl, or an alkali metal or alkaline earth metal, a is 0.0005-0.2, b is 0.3a, c is 0.5-2.5, x has a value to satisfy the valencies, and Ti and P of the crystalline TiP2O7 represent part of the crystalline TiP2O7.
BR-A-91/04652 teaches pre-treatment of a palladium on a silica carrier catalyst for production of ethyl acetate by direct oxidation of ethanol with air.
Production of esters from primary alcohols by dehydrogenation using bromous acid or a salt thereof in acid medium is described in JP-A-59/025334.
In SU-A-362814 there is described a process for production of ethyl acetate by dehydrogenation of ethanol at 180xc2x0 C. to 300xc2x0 C. in the presence of a copper catalyst containing zinc as an activator with an ethanol feed rate of 250 to 700 liters per liter of catalyst per hour.
The dehydrogenation of ethanol to form ethyl acetate is described in GB-A-287846. This proposes use of a dehydrogenating agent, such as a copper catalyst, a temperature of from 250xc2x0 C. to 500xc2x0 C., and a pressure of more than 10 atmospheres (1.013xc3x97106 Pa).
Vapour phase contact of ethanol at a temperature above its critical temperature with a catalyst comprising copper and a difficultly reducible oxide, such as zinc oxide or manganese oxide, is proposed in GB-A-312345, use of a temperature of 375xc2x0 C. and a pressure of 4000 psi (27.58 Mpa) being suggested.
GB-A-470773 teaches a process for conversion of ethanol to ethyl acetate by dehydrogenating ethanol over a catalyst consisting of a reduced metal, for example, copper on infusorial earth with 10% uranium oxide as promoter, maintained at a temperature of 220xc2x0 C. to 260xc2x0 C., removing by condensation some of the gas-vapour product rich in hydrogen resulting from the reaction, and returning the gaseous remainder rich in hydrogen to the catalysing zone.
EP-A-0151886 describes a process for the preparation of C2+ esters of alkyl carboxylic acids from C2+ primary alcohols which comprises contacting a vaporous mixture containing a primary C2+ alkanol and hydrogen in an alkanol:hydrogen molar ratio of from 1:10 to about 1000:1 at a combined partial pressure of alkanol and hydrogen of from about 0.1 bar (103 Pa) up to about 40 bar (4xc3x97106 Pa) and at a temperature in the range of from about 180xc2x0 C. to about 300xc2x0 C. in a catalytic reaction zone with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide, and recovering a reaction product mixture containing a primary C2+ alkyl ester of an alkyl carboxylic acid which ester contains twice as many carbon atoms as the primary C2+ alkanol.
In EP-A-0201105 there is described a method for converting primary alcohols, such as ethanol, to their corresponding alkanoate esters which involves the regulation of the mole feed ratio of hydrogen gas to alkanol in the reaction zone of a copper chromite containing catalyst.
One method of separating ethyl acetate from ethanol and water involves extractive distillation with an extractive agent comprising polyethylene glycol and dipropylene glycol, diethylene glycol, or triethylene glycol as described in U.S. Pat. No. 4,569,726 or with an extractive agent containing dimethyl sulphoxide as described in U.S. Pat. No. 4,379,028.
Separation of ethyl acetate from a composition comprising ethyl acetate, ethanol and water is disclosed in JP-A-05/186392 by feeding the composition to a distillation column to obtain a quasi-azeotropic mixture comprising ethyl acetate, ethanol and water, condensing it, separating the condensate into an organic layer and an aqueous layer, returning the organic layer to the column, and recovering ethyl acetate as a bottom product from the column.
EP-A-0331021 describes how carbonylation of olefins to produce monocarboxylate esters causes formation of aldehydes and acetals as byproducts. Monocarboxylate esters produced in this way are subjected to a three step purification process involving treatment with a strongly acidic agent, followed by hydrogenation and distillation. The initial treatment with a strongly acidic agent is intended to convert acetals to vinyl ethers and aldehydes and acetals to aldols. The subsequent hydrogenation step then converts these compounds to byproducts which are more easily separated from the desired monocarboxylate ester.
EP-A-0101910 contains a similar disclosure regarding carbonylation of olefins to give monocarboxylate esters. It proposes treatment of the monocarboxylate ester with hydrogen at elevated temperature in the presence of an acidic ion exchanger or zeolite doped with one or more metals of Group VIII of the Periodic Table, followed by hydrogenation. It is stated that acetals present as byproducts are converted to vinyl ethers which are converted by hydrogenation to low boiling esters or the aldehydes and acetals are converted to high boilers by an aldol reaction. Unsaturated ketones are converted to saturated ketones.
It would be desirable to provide an improved commercial method of upgrading ethanol to ethyl acetate, a more valuable product, particularly where there is an over-capacity for ethanol. It would also be desirable to provide a novel route to high purity ethyl acetate which obviates the need for a separate acetaldehyde or acetic acid plant. It would further be desirable to provide a process for the production of substantially pure ethyl acetate directly from ethanol without the need to convert part of the ethanol feedstock to acetaldehyde or to acetic acid. Additionally it would be desirable to provide a route to ethyl acetate by dehydrogenation of ethanol which is capable to yielding high purity ethyl acetate from ethanol feed streams containing significant amounts of impurities.
One particular problem in production of ethyl acetate by dehydrogenation of ethanol is that the reaction product mixture tends to be a complex mixture including esters, alcohols, aldehydes and ketones. The reaction mixture can be even more complex when the ethanol feed contains impurities. The reaction product mixtures contain components with boiling points close to ethyl acetate (such as U-butyraldehyde and butan-2-one), including components which can form azeotropes with ethyl acetate, and/or other components of the mixture. This is a particular problem when high purity ethyl acetate is desired. Another problem is that water present in the feed ethanol or produced as a by-product during dehydrogenation has a deactivating effect on dehydrogenation catalysts so that any recycle to the dehydrogenation reactor of unconverted ethanol should desirably contain only a low level, if any, of water.
The present invention accordingly seeks to provide a novel process for production of ethyl acetate from ethanol, enabling production of ethyl acetate at a relatively low cost and involving simple plant. Another object of the present invention is to provide an improved process for the production of high purity ethyl acetate from ethanol, or from a feedstock comprising a major proportion of ethanol and a minor proportion of impurities such as iso-propanol.
According to the present invention there is provided a process for the production of ethyl acetate which comprises:
(a) converting a C2 feedstock comprising ethanol to ethyl acetate in an ethyl acetate production zone by a procedure selected from:
(i) dehydrogenation,
(ii) oxidation,
(iii) reaction with acetaldehyde, and
(iv) oxidation to acetaldehyde followed by the Tischenko reaction;
(b) recovering from the ethyl acetate production zone an intermediate reaction product mixture comprising hydrogen and liquefiable products comprising ethyl acetate, ethanol, and by-products containing reactive carbonyl groups;
(c) contacting at least a portion of the liquefiable products of the intermediate reaction product mixture with a selective hydrogenation catalyst in the presence of hydrogen in a selective hydrogenation zone maintained under selective hydrogenation conditions effective for selective hydrogenation of by-products containing reactive carbonyl groups thereby to hydrogenate said by-products selectively to hydrogenated by-products comprising corresponding alcohols;
(d) recovering from the selective hydrogenation zone a selectively hydrogenated reaction product mixture comprising ethyl acetate, ethanol, hydrogen and hydrogenated by-products;
(e) distilling material of the selectively hydrogenated reaction product mixture in one or more distillation zones so as to produce a first composition comprising substantially pure ethyl acetate and a second composition comprising ethanol and water;
(f) treating the second composition of step (e) to separate water therefrom and yield a third composition comprising ethanol with a reduced water content; and
(g) recovering the third composition of step (f).
In step (a) of the process of the invention the C2 feedstock is converted to ethyl acetate by (i) dehydrogenation, (ii) oxidation, (iii) reaction with acetaldehyde, or (iv) oxidation to acetaldehyde followed by the Tischenko reaction. In all-of these processes by-products of the reaction include C4 compounds having boiling points which are close to that of ethyl acetate (b.p. 77.1xc2x0 C.) and hence give rise to problems in purification of the ethyl acetate product. Notable amongst these by-products are butane-2-one (b.p. 79.6xc2x0 C.) and n-butyraldehyde (b.p. 75.7xc2x0 C.). Such by-products are not produced in the course of production of ethyl acetate by esterification of ethanol with acetic acid.
The C2 feedstock used instep (a) comprises ethanol which has been produced by hydration of ethylene, by the Fischer Tropsch process, or by fermentation of a carbohydrate source, such as starch. It may alternatively be a byproduct of another industrial process. It may contain, besides ethanol, minor amounts of water as well as small amounts of impurities resulting from byproduct formation during its synthesis. If the C2 feedstock includes recycled unreacted ethanol, then any by-products formed in the dehydrogenation step which are contained in the recycled ethanol will also contribute to the level of by-products present in the C2 feedstock. Impurities present in the C2 feedstock may include, for example, higher alcohols such as n-propanol, iso-propanol, n-butanol and sec-pentanol; ethers, such as diethyl ether, and di-iso-propyl ether; esters, such as iso-propyl acetate, s-butyl acetate and ethyl butyrate; and ketones, such as acetone, butan-2-one, and 2-pentanone. At least some of these impurities can be difficult to remove from ethyl acetate, even when they are present in quantities as low as about 0.1 mol % or less, by traditional distillation procedures because they have boiling points which are close to that of ethyl acetate and/or form constant boiling mixtures therewith.
In step (a) the C2 feedstock may be subjected to dehydrogenation according to equation (3) above. In this case the C2 feedstock can be converted to ethyl acetate by a dehydrogenation procedure which comprises contacting a vaporous mixture containing ethanol and hydrogen with a dehydrogenation catalyst in a dehydrogenation zone maintained under dehydrogenation conditions effective for dehydrogenation of ethanol to yield ethyl acetate.
Typical dehydrogenation conditions include use of an ethanol:hydrogen molar ratio of from about 1:10 to about 1000:1, a combined partial pressure of ethanol and hydrogen of up to about 50 bar (5xc3x97106 Pa), and a temperature in the range of from about 100xc2x0 C. to about 260xc2x0 C.
Preferably the combined partial pressure of ethanol and hydrogen ranges from about 3 bar (3xc3x97105 Pa) up to about 50 bar (5xc3x97106 Pa), and is more preferably at least 6 bar (6xc3x97105 Pa) up to about 30 bar (3xc3x97106 Pa), and even more preferably in the range of from about 10 bar (106 Pa) up to about 20 bar (3xc3x97106 Pa), for example from about 12 bar (1.2xc3x97106 Pa) to about 15 bar (1.5xc3x97106 Pa).
Dehydrogenation is preferably conducted in the dehydrogenation zone at a temperature of from about 200xc2x0 C. to about 250xc2x0 C., preferably at a temperature in the range of from about 210xc2x0 C. to about 240xc2x0 C., even more preferably at a temperature of about 220xc2x0 C.
The ethanol:hydrogen molar ratio in the vaporous mixture fed into contact with the dehydrogenation catalyst usually will not exceed about 400:1 or about 500:1 and may be no more than about 50:1.
The dehydrogenation catalyst is desirably a catalyst containing copper, optionally in combination with chromium, manganese, aluminium, zinc, nickel or a combination of two or more of these metals, such as a copper, manganese and aluminium containing catalyst. Preferred catalysts comprise, before reduction, copper oxide on alumina, an example of which is the catalyst sold by Mallinckrodt Specialty Chemicals, Inc., under the designation E408Tu, a catalyst which contains 8% by weight of alumina. Other preferred catalysts include chromium promoted copper catalysts available under the designations PG85/1 (Kvaerner Process Technology Limited) and CU0203T (Engelhard), manganese promoted copper catalysts sold under the designation T4489 (Sud Chemie AG), and supported copper catalysts sold under the designation D-32-J (Sud Chemie AG). E408Tu is a particularly preferred dehydrogenation catalyst.
In the dehydrogenation step the rate of supply of the C2 feedstock to the dehydrogenation zone typically corresponds to an ethanol liquid hourly space velocity (LHSV) of from about 0.5 hrxe2x88x921 to about 1.0 hrxe2x88x921.
Hydrogen is produced as a result of the dehydrogenation reaction and can be recycled to the dehydrogenation zone from downstream in the process. The hydrogen can be substantially pure hydrogen or can be in the form of a mixture with other gases that are inert to the C2 feedstock and to the dehydrogenation catalyst. Examples of such other gases include inert gases such as nitrogen, methane and argon.
In the dehydrogenation zone, side reactions may also occur, including formation of water. It is postulated that such side reactions include formation of acetaldehyde which in turn can undergo aldol formation, followed by dehydration to form an unsaturated alcohol and water. These reactions can be summarised thus:
CH3CH2OHxe2x95x90CH3CHO+H2xe2x80x83xe2x80x83(5)
2CH3CHOxe2x95x90CH3CH(OH)CH2CHOxe2x80x83xe2x80x83(6) and
CH3CH(OH)CH2CHOxe2x95x90CH3CHxe2x95x90CHCHO+H2Oxe2x80x83xe2x80x83(7).
The crotonaldehyde produced by equation (7) can then undergo hydrogenation to form 2-butanol thus:
CH3CH=CHCHO+H2=CH3CH2CH2CH2OHxe2x80x83xe2x80x83(8).
Other side reactions which release water as a by-product include formation of ketones, such as acetone and butan-2-one, and formation of ethers, such as diethyl ether.
It is alternatively possible to subject the C2 feedstock to oxidation in order to effect production of ethyl acetate as taught by U.S. Pat. No. 5,334,751 or by U.S. Pat. No. 4,996,007.
Alternatively the C2 feedstock can be passed in admixture with air over a palladium on a silica carrier catalyst thereby to produce ethyl acetate by direct oxidation of ethanol with air as taught by BR-A-91/04652.
Yet another method of effecting oxidation of the C2 feedstock so as to produce ethyl acetate is to use bromous acid or a salt thereof in acid medium as described in JP-A-59/025334.
Yet another method of converting the C2 feedstock to ethyl acetate is to react it with acetaldehyde to yield ethyl acetate and hydrogen according to equation (4) above.
Still another method of converting the C2 feedstock to ethyl acetate involves oxidation of ethanol to acetaldehyde, for example by the process of U.S. Pat. No. 4,220,803, followed by conversion of the acetaldehyde product to ethyl acetate by the Tischenko reaction of equation (2) above. Typical Tischenko reaction conditions are set out in U.S. Pat. No. 3,714,236. Again water can be a by-product of this reaction, being formed, it is postulated, by equations (6) and (7) above.
In step (b) of the process of the invention there is recovered from the ethyl acetate production zone an intermediate reaction product mixture comprising hydrogen and liquefiable products comprising ethyl acetate, ethanol, water, and by-products containing reactive carbonyl groups. This step can be effected in any convenient manner and may include a condensation step in order to condense liquefiable products present in the intermediate reaction product mixture. Alternatively the intermediate reaction product can be passed directly to step (c) without any intermediate condensation step.
A range of undesirable by-products may be present in the intermediate reaction product mixture, some of which would cause separation problems if the intermediate reaction product mixture were to be directly refined because their boiling points are close to that of ethyl acetate or because they form azeotropes with ethyl acetate whose boiling point is close to that of ethyl acetate. Such by-products may be present in the C2 feedstock or may be produced in step (c). Problematical by-products are aldehydes and ketones, such as n-butyraldehyde and butan-2-one. In order to avoid problems due to the presence of such by-products in the distillation step (e), even in amounts as small as about 0.1 mol % or less, e.g. about 0.01 mol % or less, the problematical by-products are substantially removed as a result of the selective hydrogenation step (c). Accordingly, liquefiable products present in the intermediate reaction product mixture of step (b) are reacted in step (c) with hydrogen over a suitable selective hydrogenation catalyst. The catalyst type and reaction conditions are chosen so that aldehydes and ketones are hydrogenated to their respective alcohols, while hydrogenation of ethyl acetate is minimal. Among aldehyde and ketone by-products which may be present, butan-2-one and n-butyraldehyde, in particular, would otherwise cause problems in any subsequent distillation. These compounds are hydrogenated in the selective hydrogenation zone in step (c) to the corresponding alcohols, i.e. 2-butanol and n-butanol respectively, which can be readily separated from ethyl acetate by distillation.
The mixture supplied to the selective hydrogenation zone in step (c) contains, in addition to ethanol, hydrogen either alone or in admixture with one or more inert gases that are inert to the reactants and catalysts in the selective hydrogenation step (c) of the process of the invention. Examples of such inert gases have been given above. The source of the hydrogen used in the selective hydrogenation step (c) may be hydrogen formed in the dehydrogenation step and may include gas recycled from the downstream end of the selective hydrogenation zone.
The selective hydrogenation step (c) is typically conducted at a temperature of from about 20xc2x0 C. to about 160xc2x0 C., preferably at a temperature in the range of from about 40xc2x0 C. to 120xc2x0 C., even more preferably at a temperature of about 60xc2x0 C. to about 80xc2x0 C. Typical selective hydrogenation conditions include use of a reaction product mixture:hydrogen molar ratio of from about 1000:1 to about 1:1, preferably from about 100:1 to about 5:1, for example about 20:1.
The combined partial pressure of liquefiable products and hydrogen in the selective hydrogenation zone typically lies in the range of from about 5 bar (5xc3x97105 Pa) up to about 80 bar (8xc3x97106 Pa), and is even more typically from about 25 bar (2.5xc3x97106 Pa) to about 50 bar (5xc3x97106 Pa).
The selective hydrogenation catalyst used in step (c) of the process of the invention is selected to have good activity for hydrogenation of reactive carbonyl containing compounds, but relatively poor ester hydrogenation activity. Suitable catalysts comprise metals selected from nickel, palladium and platinum. Ruthenium, supported on carbon, alumina or silica is also effective, as are other metal catalysts such as rhodium and rhenium. Preferred catalysts include nickel on alumina or silica and ruthenium on carbon. Particularly preferred catalysts include 5% ruthenium on carbon available from Engelhard.
The rate of supply of liquefiable liquid products of the intermediate reaction product mixture to the selective hydrogenation zone depends upon the activity of the selective hydrogenation catalyst but typically corresponds to a liquid hourly space velocity (LHSV) of from about 0.1 hrxe2x88x921 to about 2.0 hrxe2x88x921, preferably from about 0.2 hrxe2x88x921 to about 1.5 hrxe2x88x921. When using, for example, a ruthenium on carbon catalyst the LHSV may be from about 0.5 hrxe2x88x921 to about 2.0 hrxe2x88x921, for example from about 1.0 hrxe2x88x921 to about 1.5 hrxe2x88x921. When using a nickel containing catalyst the LHSV may be, for example, from about 0.3 hrxe2x88x921 to about 0.5 hr31 1.
Step (d) of the process of the present invention comprises recovering from the selective hydrogenation zone a selectively hydrogenated reaction product mixture comprising ethyl acetate, ethanol, hydrogen and hydrogenated by-products. Typically this includes a condensation step in order to separate liquefiable materials from a gaseous stream containing unreacted hydrogen which can be recycled for dehydrogenation or for selective hydrogenation.
Step (e) of the process of the invention comprises distilling material of the selectively hydrogenated reaction product mixture in one or more distillation zones so as to produce a first composition comprising substantially pure ethyl acetate and a second composition comprising ethanol and water. In this step the material subjected to distillation typically has a water content of less than about 20 mol %, more usually not more than about 15 mol %.
Ethanol, water and ethyl acetate form a minimum boiling ternary azeotrope upon distillation thereof.
Step (e) may comprise an extractive distillation procedure as described in U.S. Pat. No. 4,569,726 or in U.S. Pat. No. 4,379,028.
Preferably, however, distillation is carried in step (e) by a procedure which takes advantage of the fact that the composition of the minimum boiling ternary azeotrope formed by ethanol, water and ethyl acetate depends upon the pressure at which distillation is effected. Hence a preferred distillation procedure comprises supplying material of the selectively hydrogenated reaction product mixture to a first distillation zone maintained under distillation conditions effective for distillation therefrom of a first distillate comprising ethyl acetate, ethanol, and water, recovering a first distillate comprising ethyl acetate, ethanol, and water from the first distillation zone and a bottom product comprising ethanol and water, supplying material of the first distillate to a second distillation zone maintained under distillation conditions effective for distillation therefrom of a second distillate comprising ethanol, water, and ethyl acetate (typically a minor amount of ethyl acetate) and so as to yield a substantially pure ethyl acetate bottom product, and recovering a substantially pure ethyl acetate bottom product from the second distillation zone. The first distillation zone is preferably operated at a pressure less than about 4 bar (4xc3x97105 Pa), preferably from about 1 bar (105 Pa) up to about 2 bar (2xc3x97105 Pa), while the second distillation zone is operated at a higher pressure than that of the first distillation zone, for example at a pressure of from about 4 bar (4xc3x97105 Pa) to about 25 bar (2.5xc3x97106 Pa), preferably from about 9 bar (9xc3x97105 Pa) to about 15 bar (15xc3x97105 Pa).
It can be shown that in this preferred distillation procedure the rate of flow of the first distillate from the first distillation zone to the second distillation zone and the corresponding flow rate from the second distillation zone to the first distillation zone of the second distillate can be minimised by operating one of the distillation zones so that the distillate has a composition very close to that of the ternary azeotrope at that pressure. However, in order to operate that zone so that the distillate has a composition close to that of the ternary azeotrope at its pressure of operation, a high degree of separation is required which necessitates use of a column with many distillation trays and a high heat input. In addition, since water has the highest latent heat of vaporisation out of the three components of the ternary azeotrope, the total heat input to the two zones can be minimised by minimising the water content of the feeds to the distillation zones.
In addition to forming a ternary azeotrope, the three components of the ternary azeotrope can also form binary azeotropes with one of the other components. For example, ethanol forms a binary azeotrope with water and also with ethyl acetate. It is preferred to select a pressure of operation of the second distillation zone so that the binary azeotrope between ethanol and ethyl acetate at that pressure has a lower ethyl acetate content than the ternary azeotrope at that pressure and further to select a pressure of operation for the first distillation zone and to adjust the flow rates of the distillates between the first and second zones so that the first distillate has as low a water content as possible. In this way the second distillate recovered from the second distillation zone will have a low content of ethyl acetate.
In the preferred distillation procedure an ethanol rich stream containing substantially all of the water in the selectively hydrogenated reaction product mixture is recovered from the bottom of the first distillation zone, while an overhead stream that contains xe2x80x9clightxe2x80x9d components present in the selectively hydrogenated reaction product mixture is recovered from the first distillation zone, and the first distillate comprises a liquid draw stream which is recovered from an upper region of the first distillation zone and which comprises ethyl acetate, ethanol, water and minor amounts of other components. By the term xe2x80x9clightxe2x80x9d components is meant components that have lower boiling points than ethyl acetate and its azeotropes with water and ethanol. The liquid draw stream typically contains less than about 10 mol % water. For example, it suitably comprises from about 1 mol % to about 6 mol % water, from about 40 mol % to about 55 mol % ethyl acetate, not more than about 2 mol % minor products (preferably not more than about 1 mol % minor products) and the balance ethanol. Thus it may typically contain about 45 mol % ethyl acetate, about 50 mol % ethanol, about 4 mol % water and about 1 mol % other components. This liquid draw stream is passed to the second distillation zone. The second distillate, with a typical composition of about 25 mol % ethyl acetate, about 68 mol % ethanol, about 6 mol % water, and about 1 mol % other components, is recovered as an overhead stream from the second distillation zone, while a bottom product comprising ethyl acetate is recovered from the second distillation zone which typically contains from about 99.8 mol % to about 99.95 mol % ethyl acetate; this second distillate is returned to the first distillation zone, preferably at a point above the feed point of the liquefiable products of the selectively hydrogenated reaction product mixture.
The overhead stream from the first distillation zone contains xe2x80x9clightxe2x80x9d components present in the intermediate reaction product mixture, such as diethyl ether, acetaldehyde and acetone. It can be burnt as a fuel.
In step (f) of the process of the invention the ethanol rich stream recovered from the bottom of the first distillation zone is subjected to treatment for the removal of water therefrom thereby to produce a relatively dry ethanol stream which is suitable for recycle to step (a), if desired. This ethanol rich stream will contain any xe2x80x9cheaviesxe2x80x9d, i.e. products, including unknown products, with high boiling points compared to those of ethanol and ethyl acetate. These can be separated from the ethanol and water by distillation, if desired, prior to effecting removal of water from the resulting distillate. One suitable method for removal of water from the ethanol rich stream or from the distillate resulting from xe2x80x9cheaviesxe2x80x9d removal is molecular sieve adsorption. Azeotropic distillation with a suitable entrainment agent, such as benzene or cyclohexane, can alternatively be used. Membranes are currently under development which will enable separation of water from ethanol; these are reported to be nearly ready for commercial exploitation. Hence use of a membrane is another option available for separating water from the ethanol rich stream.
Preferably the water content of the relatively dry ethanol produced in step (f) is less than about 5 mol %, and preferably less than about 2 mol %.